The conventional mechanisms for effecting a resistance are generally attained by means of mechanical friction, oil pressure, vortex, electrical machine, etc. The friction resistance device is incapable of controlling the magnitude of resistance with precision and is unstable because of the problems of wear and load. The oil pressure resistance device has problems of oil leakage, noise, and load reduction. The electrical machine resistance device is complicated in construction and very expensive. The vortex resistance device makes use of a permanent magnet as a magnetic field source, or an electromagnetic coil as a magnetic field source. The permanent magnet resistance device is defective in design in that it can not be easily connected with an external communication signal for controlling the magnitude of the resistance force. The electromagnetic coil resistance device can be connected with the external digital or analog signal for controlling the magnitude of the magnetic field resistance. As shown in FIG. 1, the electromagnetic coil resistance device comprises an electromagnetic coil 4' winding around a U-shaped electromagnetic core 3', which is provided with a space P' for receiving the wheel rim of the braking wheel 2'. When the coil is provided with power, the current passes the coil to form a magnetic field. The magnetic flux is brought about on the iron core 3'. In the meantime, the rim of the braking wheel 2' passes the space P' of the opening of the iron core 3', the eddy current is brought about such that the eddy current forms a resistance of reverse rotation moment as a load to the braking wheel 2'. The electromagnetic coil structure is defective in design in that the point of application force of an entire resistance force is smaller to reach only the small area of the space P' of the opening of the U-shaped iron core, and that its magnetic loop structure tends to bring about the magnetic leakage, which results in an ineffective control of resistance exerting on the braking wheel.